Differential with outwardly directed planetary gear separating forces

ABSTRACT

A parallel-axis gear differential (10) includes a planetary gearing assembly of sun gears (22 and 24) and planet gears (26, 28, 30, 32, 34, and 36) carried within a housing (12). The planet gears (26, 28, 30, 32, 34, and 36), which are mounted in pairs within pockets (38, 40, 42, 44, 46, and 48) formed in the housing (12), include a first point of meshing engagement (74, 76) with one of the side gears (22, 24) and second and third points of meshing engagement (78 and 80) with a paired planet gear. A radial load component &#34;Ws1&#34; at the first point of engagement (74, 76) is adjusted with respect to a combined radial load component &#34;Ws2&#34; of the second and third points of engagement (78 and 80) for maintaining the planet gears (26, 28, 30, 32, 34, and 36) in their proper running positions within pockets (38, 40, 42, 44, 46, and 48).

TECHNICAL FIELD

The invention relates to the field of differentials having planetarygear sets for interconnecting a pair of output shafts.

BACKGROUND

Gear differentials include compound planetary gear sets carried within ahousing for interconnecting a pair of output shafts. The planetary gearsets permit the output shafts to rotate in opposite directions withrespect to the housing. An input shaft is operatively connected to thehousing for rotating the housing about a common axis of the outputshafts.

Sun gear members of the planetary gear sets, also referred to as "side"gears, are coupled to inner ends of the output shafts. Planet gearmembers of the same sets, also referred to as "element" gears,operatively connect the side gears for relative rotation in oppositedirections. The sun gear members rotate together with the output shaftsabout the common axis of the output shafts. However, the planet gearsrotate about axes that can be variously offset and inclined with respectto the common axis.

One known type of gear differential, referred to as a "parallel-axis"gear differential, includes the sun and planet gears mounted about axesthat extend parallel to each other. The planet gears of this type ofdifferential are generally mounted in pairs within the housing. Oneportion of each planet gear meshes with one of the side gears, andanother portion of each planet gear meshes with its paired planet gear.

The planet gears are individually supported for rotation on shafts orwithin pockets formed in the housing. The shafts are journalled in boresformed in the housing at opposite ends of the planet gears. The pocketsprovide bearings for slidably supporting outside cylindrical surfaces ofthe planet gears formed by top lands of the planet gear teeth.

One example of a parallel-axis gear differential having planet gearsindividually supported within housing pockets is disclosed in U.S. Pat.No. 3,706,239 (MYERS). The pockets, together with other gear mountingsurfaces in the differential of Myers, provide frictional surfaces foropposing relative rotation (i.e., differentiation) of the planet gearset. The amount of friction is proportional to the total drive torquetransmitted by the differential housing, and the friction is used tosupport torque differences between the output shafts.

Differentials that develop a frictional resistance to differentiationproportional to drive torque (like the one disclosed in Myers) arereferred to as "torque proportioning" differentials. The frictionalresistance helps to compensate for an uneven traction conditionpresented to a pair of drive wheels by delivering more drive torque tothe wheel with better traction. In turns, more drive torque is deliveredto the slower rotating drive wheel.

Gear tooth forces acting at two different locations on the planet gearsin Myers tend to misalign or tilt the planet gears within their pockets.Although some misalignment of the planet gears can be used to generateincreased frictional resistance to differentiation, a spacer is requiredto provide additional radial support for the planet gears. The spacerincludes arcuate segments for enclosing openings in the pockets betweenthe side gears.

Another example of a parallel-axis gear differential having planet gearsmounted within housing pockets is disclosed in U.S. Pat. No. 5,122,101(TSENG), a patent commonly assigned herewith. The planet gears areformed as so-called "combination" gears having two gear sectionsseparated by a stem. A first of the gear sections of each combinationgear includes respective points of mesh with one of the side gears andwith a second of the gear sections of a paired combination gear. Thesecond section of each combination gear includes a point of mesh withthe first portion of its paired combination gear. The two points of meshbetween paired combination gears straddle points of mesh between thepaired combination gears and the side gears along the common axis of theside gears.

Although the two points of mesh between paired combination gears reducethe tendency of the combination gears to tilt within their pockets,movement of the combination gears toward the common axis can produceinterference with teeth of the side gears. The interference can becaused by either angular or rectilinear gear movements that reducebacklash between the combination gears and the side gears. The loss ofbacklash can cause the mating gear teeth of the gears to bind together,thereby increasing gear tooth wear and possibilities for gear toothfailure and producing inconsistent differential performance.

SUMMARY OF INVENTION

The invention involves improvements to parallel-axis gear differentialshaving planet gears mounted within housing pockets. The differentialsare configured similar to the differentials disclosed in U.S. Pat. No.5,122,101 (TSENG), and particular relationships are observed in thedesign and dimensioning of the gearing to resist potentially damagingmovements of the gears out of their desired running positions.

One example of the improved differential includes a housing that isrotatable by drive torque about a common axis of a pair of outputshafts. A pair of side gears is positioned in the housing for rotationwith the output shafts about the common axis. One or more pairs ofcombination gears are positioned within the housing forming separategear trains for operatively connecting the side gears. An equal numberof pairs of pockets are formed in the housing supporting the pairs ofcombination gears for rotation about respective axes that extendparallel to the common axis.

Each combination gear has a first point of meshing engagement with oneof the side gears and second and third points of meshing engagement withits paired combination gear. The second point of meshing engagement islocated at a first distance "a" along the common axis from the firstpoint of meshing engagement, and the third point of meshing engagementis located at a second distance "b" in an opposite direction along thecommon axis from the first point of meshing engagement. The firstdistance "a" is no larger than the second distance "b".

A first radial load transmitted by the first point of meshing engagementincludes a component "W_(s1) " along a line of centers between thecommon axis and the axis of the combination gear in mesh at the firstpoint of meshing engagement. A second radial load divided between thesecond and third points of meshing engagement includes a component"W_(s2) " in an opposite direction along the line of centers. Thepotentially damaging movements of the combination gears are resisted byrelating the components "W_(s1) " and "W_(s2) " of the first and secondradial loads according to the following inequality:

    2a W.sub.s1 ≧(a+b)W.sub.s2

The two force components "W_(s1) " and "W_(s2) " are functions of atransverse pressure angle "phi_(t) " between mating gear tooth surfacesat the three points of meshing engagement. A transverse pressure angle"phi_(t) " of sufficient magnitude to satisfy the above relationshipbetween force components "W_(s1) " and "W_(s2) " is given by thefollowing inequality: ##EQU1## where "theta" is defined as an angleformed between the line of centers and a radial line extendingperpendicular to the common axis through the second and third points ofmeshing engagement projected into a transverse plane of the side andcombination gears.

DRAWINGS

FIG. 1 is perspective view of the type of differential treated by theinvention with portions of a housing removed to reveal planetarygearing.

FIG. 2 is a cross-sectional view of the differential in FIG. 1 takenalong line 2--2.

FIG. 3 is a diagrammatic representation of the planetary gearing shownin FIG. 1 with the individual gears rotated from the mounting positionsinto a common axial plane.

FIG. 4 is an enlarged diagrammatic representation of the same planetarygears as they would appear in their correct mounting positions viewed ina transverse plane similar to FIG. 2.

DETAILED DESCRIPTION

Illustrated by FIGS. 1 and 2 is a parallel-axis gear differential 10similar to the differential disclosed in commonly assigned U.S. Pat. No.5,122,101. The disclosure of this commonly assigned patent is herebyincorporated by reference. The differential 10 has a housing 12 thatreceives and supports the ends of two output shafts 14 and 16 forrotation about a common axis 18. A flange 20, formed integrally with thehousing 12, is adapted to receive a ring gear (not shown) fortransmitting drive power to the housing 12.

Coupled to the ends of the output shafts 14 and 16 are side gears 22 and24 that function as sun gears within a planetary gear arrangement. Threepairs of combination gears 26 and 28, 30 and 32, and 34 and 36, whichfunction as planet gear pairs, are positioned within the housing 12forming separate gear trains for rotating the side gears 22 and 24 inopposite directions about the common axis 18. Although the illustrateddifferential 10 includes three pairs of combination gears, more or fewerpairs of combination gears can be used to provide a similar connectionbetween the side gears.

The individual combination gears 26, 28, 30, 32, 34, and 36 are mountedfor rotation within respective pockets 38, 40, 42, 44, 46, and 48 thatform bearing surfaces within the housing 12 for supporting outsidecylinder surfaces of the combination gears. The pockets 38, 40, 42, 44,46, and 48 are positioned within the housing 12 for supporting rotationsof the combination gears 26, 28, 30, 32, 34, and 36 about respectiveaxes 50, 52, 54, 56, 58, and 60 that extend parallel to the common axis18.

The combination gears 26, 30, and 34 mesh with the side gear 22; and thecombination gears 28, 32, and 36 mesh with the side gear 24. Thecombination gears 26 and 28 of a first pair, the combination gears 30and 32 of a second pair, and the combination gears 34 and 36 of a thirdpair also mesh with each other at two different locations. The meshingrelationships of the combination gears 26 and 28 comprising one of thegear trains interconnecting the side gears 22 and 24 are illustrated byFIG. 3 in which, for purposes of simplification, the gear train has beenunwrapped to show all of the gear axes 18, 50, and 52 in a single plane,the common axis 18 being split as indicated.

Each of the combination gears 26 and 28 includes two gear sectionsseparated by a stem section. For example, the combination gear 26includes: a first gear section 62 in mesh with both the side gear 22 anda second gear section 70 of its paired combination gear 28, a secondgear section 64 in mesh with a first gear section 68 of its pairedcombination gear 28, and a stem section 66 for providing clearance withthe side gear 24. The first gear section 68 of the combination gear 28also meshes with the side gear 24, and a stem section 72 of the samecombination gear provides clearance with the side gear 22.

Each of the combination gears 26 and 28 also includes three points ofmeshing engagement (i.e., points of contact). For example, thecombination gears 26 and 28 have respective first points of meshingengagement 74 and 76 with the side gears 22 and 24 and have respectivesecond and third points of meshing engagement 78 and 80 with each other.The first points of meshing engagement 74 and 76 are located midway ofthe side gear face widths. The second and third points of meshingengagement 78 and 80 are located midway of the second gear section facewidths.

Transverse planes 82, 84, 86, and 88 appear on edge in FIG. 3intersecting the respective points of meshing engagement 78, 74, 76, and80. The second and third points of meshing engagement 78 and 80 arelocated at respective distances "a₁ " and "b₁ " along the common axis 18from the first point of meshing engagement 74 and at respectivedistances "b₂ " and "a₂ " along the common axis 18 from the other firstpoint of meshing engagement 76. For sake of simplicity, the distances"a₁ " and "a₂ " or an average of these distances can be considered equalto a distance "a. "Similarly, the distances "b₁ " and "b₂ " or anaverage of these distances can be considered equal to a distance "b. "

FIG. 4 depicts the side gear 24 and the two combination gears 26 and 28as their equivalent pitch circles having respective radii "r₁ " and "r₂", the pitch radii "r₂ " of the combination gears being equal. A line ofcenters 78 passes through the common axis 18 and the axis 50 of thecombination gear 26. A radial line 92 extends from the common axis 18through the second and third points of meshing engagement 78 and 80,which are projected as a common pitch point into the transverse drawingplane. An angle "theta" is defined between the line of centers 90 andthe radial line 92 and can be calculated as follows: ##EQU2##

For simplicity of illustration, a single line of action 94 is drawnthrough the first point of meshing engagement 74 (which is alsoprojected as a pitch point) and the second and third points of meshingengagement 78 and 80. However, the inclinations of the mating toothsurfaces 96 and 98 to their respective tangent planes are referenced bydifferent transverse pressure angles "phi_(t1) " and "phi_(t2) ".

Gear tooth loads communicated between the mating tooth surfaces 96 and98 produce radial loads "W_(r1) " and "W_(r2) " acting on thecombination gear 26. The two radial loads "W_(r1) " and "W_(r2) " arereferenced at the axis 50 of the combination gear 26 and are directed inopposite directions along the line of action 94. The radial load "W_(r1)" has a component "W_(s1) " along the line of centers 90, and thiscomponent "W_(s1) " is determined as a function of the transversepressure angle "phi_(t1) " as follows:

    W.sub.s1 =W.sub.r1 SIN(phi.sub.t1)

The radial load "W_(r2) " can also be considered as having a component"W_(s2) " along the same line of centers 90. However, the component"W_(s2) " is determined as a function of both the transverse pressureangle "phi_(t2) " and the angle "theta" as follows:

    W.sub.s2 =W.sub.r2 COS(phi.sub.t2 +theta)

Preferably, the component "W_(s1) " is larger than the component "W_(s2)" so that the total load acting along the line of centers 90 urges thecombination gear 26 away from the side gear 22 into engagement with itspocket 38. However, magnitudes of the component "W_(s1) " just largerthan magnitudes of the component "W_(s2) " are not necessarilysufficient to prevent tipping of the combination gear 26 about an axisthat is perpendicular to both the common axis 18 and the line of centers90.

Since the combination gears 26 and 28 have two points of meshingengagement 78 and 80, the component "W_(s2) " is divided between the twopoints of mesh within the respective transverse planes 82 and 88 shownin FIG. 3. Although different divisions of force are possible, thecomponent "W_(s2) " is expected, on average, to be divided equallybetween the two transverse planes 82 and 88. The equal division ofcomponent "W_(s2) " at the point of mesh 80 exerts a moment "M_(ab) " onthe combination gear 26 at the point of mesh 78 equal to the followingproduct with the distances "a" and "b": ##EQU3##

However, the component "W_(s1) " is effective for opposing the moment"M_(ab) " over a shorter distance "a. "An opposing moment "M_(a) " isdetermined by the following equation:

    M.sub.a =a W.sub.s1

Accordingly, to resist tipping of the combination gear 26 about an axisthat is perpendicular to both the common axis 18 and the line of centers90, the magnitude of the moment "M_(a) " should be not less than themagnitude of the moment "M_(ab) ", a relationship that can otherwise beexpressed by the following inequality:

    2a W.sub.s1 ≧(a+b)W.sub.s2

Neglecting friction acting on the combination gear 26, a transversepressure angle "phi_(t1) " and "phit₂ ", can be determined in accordancewith the following inequality: ##EQU4##

A minimum value for the transverse pressure angle "phi_(t) ", whendistances "a " and "b " are equal, can be determined as follows:##EQU5##

The above-described relationships relating to the practice of theinvention provide a general guide for resisting potentially damagingmovements of the combination gears out of their desired runningpositions within their pockets. However, those of skill in the art willappreciate that other factors including friction may affect the runningpositions of the combination gears, and the above relationships may befurther developed to account for these factors.

We claim:
 1. A differential assembly comprising:a housing rotatable bydrive torque about a common axis of a pair of output shafts; a pair ofside gears positioned within said housing for rotation with said outputshafts about said common axis; at least one pair of combination gearspositioned within said housing for operatively connecting said sidegears; at least one pair of pockets formed in said housing supportingsaid pair of combination gears for rotation about respective axes thatextend parallel to said common axis; one of said combination gearshaving a first point of meshing engagement with one of said side gearsand second and third points of meshing engagement with its pairedcombination gear; said second point of meshing engagement being locatedat a first distance "a" along said common axis from said first point ofmeshing engagement, said third point of meshing engagement being locatedat a second distance "b" in an opposite direction along said common axisfrom said first point of meshing engagement, and said first distance "a"being not larger than said second distance "b; " said first point ofmeshing engagement being arranged for transmitting a first radial load"W_(r1) " that includes a component "W_(s1) " along a line of centersbetween said common axis and the respective axis of said one combinationgear; said second and third points of meshing engagement being arrangedfor transmitting a combined second radial load "W_(r2) " that includes acomponent "W_(s2) " in an opposite direction along said line of centers;and said components "W_(s1) " and "W_(s2) " of the first and secondradial loads being related by the following inequality:

    2a W.sub.s1 ≧(a +b)W.sub.s2


2. The differential assembly of claim 1 in which said component "W_(s1)" of the first radial load is determined in accordance with thefollowing relationship:

    W.sub.s1 =W.sub.r1 SIN(phi.sub.t1)

where "phi_(t1) " is a transverse pressure angle of mating gear toothsurfaces at said first point of meshing engagement.
 3. The differentialassembly of claim 2 in which said component "W_(s2) " of the secondradial load is determined in accordance with the following relationship:

    W.sub.s2 =W.sub.r2 COS(phi.sub.t2 +theta)

where "phi_(t2) " is an average transverse pressure angle of mating geartooth surfaces at said second and third points of meshing engagement,and "theta" is an angle formed between said line of centers and a radialline connecting said common axis to said second and third points ofmeshing engagement projected into a transverse plane that extends normalto said common axis.
 4. The differential assembly of claim 3 in which atransverse pressure angle "phi_(t) " is an average of the two transversepressure angles "phi_(t1) " and "phi_(t2) " and is related to the angle"theta" according to the following inequality: ##EQU6##
 5. Thedifferential assembly of claim 4 in which the transverse pressure angle"phi_(t) " is further related to the angle "theta" according to thefollowing inequality: ##EQU7##
 6. The differential assembly of claim 1in which each of said combination gears includes two gear sectionsseparated by a stem section, and said component "W_(s1) " of the firstradial load is of sufficient magnitude to urge both of said gearsections of said one combination gear into respective frictionalengagements with one of said pockets.
 7. The differential assembly ofclaim 6 in which said component "W_(s1) " of the first radial load is ofsufficient magnitude to prevent said one combination gear from tippingwithin its pocket about an axis that extends perpendicular to both saidline of centers and said common axis.
 8. A parallel-axis geardifferential comprising:a housing rotatable by drive torque about acommon axis of a pair of output shafts; a pair of side gears positionedwithin said housing and adapted to receive respective ends of saidoutput shafts for rotation therewith about said common axis; at leastone pair of combination gears positioned within said housing forrotation about respective axes that extend parallel to said common axis;one of said combination gears having a first point of meshing engagementwith one of said side gears and second and third points of meshingengagement with its paired combination gear; a line of centers extendingthrough both said common axis and the axis of said one combination gearin a transverse plane normal to said common axis; a radial lineextending through both said common axis and at least one of said secondand third points of meshing engagement of said one combination gearprojected into said transverse plane; an angle "theta" formed betweensaid line of centers and said radial line; and an average transversepressure angle "phi_(t) " between mating gear tooth surfaces at saidthree points of meshing engagement being determined in accordance withthe following inequality: ##EQU8##
 9. The differential of claim 8 inwhich said one side gear has a first pitch radius "r₁ ", said onecombination gear has a second pitch radius "r₂ ", and said angle "theta"is related to said first and second pitch radii "r₁ " and "r₂ " asfollows: ##EQU9##
 10. The differential of claim 9 in which said secondpoint of meshing engagement is located at a first distance "a" alongsaid common axis from said first point of meshing engagement, said thirdpoint of meshing engagement is located at a second distance "b" in anopposite direction along said common axis from said first point ofmeshing engagement, and said first distance "a" is smaller than saidsecond distance "b".
 11. The differential of claim 10 in which saidtransverse pressure angle "phi_(t) " is further limited by the followinginequality: ##EQU10##
 12. The differential of claim 8 in which a firstradial load "W_(r1) " transmitted by said first point of meshingengagement includes a component "W_(s1) " along said line of centers anda combined second radial load "W_(r2) " transmitted by said second andthird points of meshing engagement includes a component "W_(s2) " in anopposite direction along said line of centers.
 13. The differential ofclaim 12 in which said component "W_(s1) " of the first radial load isdetermined in accordance with the following relationship:

    W.sub.s1 =W.sub.r1 SIN(phi.sub.t)


14. The differential of claim 13 in which said component "W_(s2) " ofthe combined second radial load is determined in accordance with thefollowing relationship:

    W.sub.s2 =W.sub.r2 COS(phi.sub.t +theta)


15. The differential of claim 14 in which said component "W_(s1) " ofthe first radial load applies a first moment "M_(a) " at said secondpoint of engagement as follows:

    M.sub.a =a W.sub.s1


16. The differential of claim 15 in which said component "W_(s2) " ofthe combined second radial load applies a second oppositely directedmoment "M_(ab) " at said second point of engagement as follows:##EQU11##
 17. The differential of claim 16 in which said first moment"M_(a) " is larger than said second moment "M_(ab) ".